Acute kidney injury (AKI) induced by ischemia (loss of blood flow) is a significant clinical problem associated with high morbidity and mortality. Currently there are no FDA approved treatments for AKI. Increased accumulation and activation of immune system cells in the kidney after ischemia-reperfusion injury (IRI) are major mediators of AKI. Ischemic preconditioning (IPC) is a phenomenon induced by brief ischemia and reperfusion that renders an organ tolerant to subsequent prolonged ischemia. Several recent studies have demonstrated that renal IPC is in part mediated by cells of the immune system, but the precise mechanisms of the immune-mediated protection are not currently known. Preliminary studies in our laboratory have revealed that there are significant changes in kidney leukocytes induced by IPC, including a decrease in the number and activation of invariant natural killer T (iNKT) cells in the kidney after IRI. These iNKT cells have recently been shown to mediate kidney IRI through the early production of inflammatory cytokines and recruitment of neutrophils. Importantly, studies in other models have demonstrated that iNKT cells become anergic after initial stimulation;unable to respond to subsequent stimulus signals. In addition, we observed an increase in the number of resident regulatory T cells (Tregs) induced by IRI. Tregs have immunosuppressive properties and prevent excessive immune responses. The roles of iNKT cell anergy and Tregs in IPC have never been investigated. Thus we hypothesize IRI-induced iNKT cell anergy and/or accumulation of Tregs in the kidney mediate the protection afforded by IPC and propose to investigate this hypothesis through the following specific aims: 1) Test the hypothesis that kidney IPC induces phenotypic and quantitative changes in renal leukocytes, 2) Test the hypothesis that ischemia-reperfusion induced iNKT cell anergy mediates kidney IPC, 3) Test the hypothesis that regulatory T cells mediate the protective effects of kidney IPC. Relevance: Through the use of state of the art immunological methods we hope to harness immunological concepts that undergird renal IPC and identify potential novel therapeutic targets to prevent or attenuate the occurrence of AKI in high-risk patients. These strategies will likely be generalizable to IPC of other organs.